1. Field of the Invention
This invention relates to improved event position detection circuitry for use in a radiation detector, such as a scintillation camera for detecting gamma rays.
2. Description of the Prior Art
Radiation detectors are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the nuclear medical diagnosis of a human body organ. A typical radiation detector of a type to which the present invention relates is a commercial version of the Anger-type scintillation camera, the basic principles of which are described in Anger U.S. Pat. No. 3,011,057.
Such a scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. As individual gamma rays are emitted from the distributed radioactivity in the object and pass through a collimator, they produce scintillation events in a thin planar scintillation crystal. The events are detected by photodetectors positioned behind the crystal. Electronic circuitry translates the outputs of the photodetectors into X and Y coordinate signals which indicate the position in the crystal of each event and a Z signal which indicates generally the energy of the event and is used to determine whether the event falls within a preselected energy window. A picture of the radioactivity distribution in the object may be obtained by coupling the X and Y signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals. The detection circuitry typically provides for integrating a large number of spots onto photographic film.
The "resolution" of a scintillation camera refers to the degree of ability of the camera faithfully to reproduce the spatial distribution of the radioactivity which is within the field of view of the device. The overall intrinsic resolution of the Anger camera detector is generally dependent on the ability of the detector to signal accurately the position coordinates of each scintillation event. There are many operations involved in the detection of each scintillation event and the signaling of its position coordinates.
As described in Arseneau U.S. Pat. No. 3,984,689, the detection circuitry of a typical prior art scintillation camera comprises a plurality of resistors joined into a positioning matrix and connected to receive the electrical pulse outputs of the photodetectors in response to the scintillation events caused in the crystal by incident radiation. Positional signals are developed by the matrix that reflect the instantaneous voltage picture in a two-dimensional rectilinear coordinate system of the position of a particular scintillation event. These signals from the positioning matrix are transferred to integrators which develop signals over a predetermined integrating time based on the associated quanta of radiation incident on the scintillation crystal.
Storage buffers are connected to the integrators to hold the integrated signals for further processing, such as by an energy correction ratio computation circuit and orientation circuits which provide horizontal and vertical deflection of a cathode ray beam.
The integrators connected to receive the outputs of the positioning matrix suffer a certain "input dead time" subsequent to the processing time for an event pulse. When the integrator is busy, holding or transferring signals to the buffer stage, the integrator is unavailable for processing subsequent pulses from the positioning matrix corresponding to further scintillation events. This "input dead time" restricts the count rate of the pulse processing system.
Reducing the "input dead time" to improve count rate by shortening the integration time is unsatisfactory. Reducing the integration time causes a deterioration of the resolution of the scintillation camera. In order to get accurate representative event positioning signals, it is necessary to consider the changes in instantaneous value of a positioning signal over a sufficiently long period of time. To reduce this time period so that integrating time is reduced, causes the resulting values of the positioning signals to be less representative and increases the probability of error.